Improved spill control is a cost effective way to reduce effluent discharge and lower the cost of waste water treatment.  The most significant spills in a mill occur from black, white and green liquors, and from paper coatings. Planned discharges include cooling water, boiler blow down, bleaching filtrates and wet debarking effluents. These discharges are generally, predictable and can be established by process simulation tools. Mills usually have spill containment systems for unplanned discharges such as leaks, tank overflows, and off-spec product dumps. In a modern mill with closed screening, O2 delignification and ECF bleaching, spills can account for up to half the colour in the final effluent, and up to a third of the operating cost of the waste treatment system [1]. Reducing spills will thus allow a mill to expand without expanding its effluent treatment system. 

Specific conductivity is the most widely used parameter for the continuous monitoring of black, white and green liquors, and soap spills. However, conductivity measurements will not detect paper coatings or turpentine spills. COD and colour are useful parameters for longer term assessment.  Spills have to be monitored instantaneously by conductivity or other real-time continuous sensors, at multiple points, to enable the appropriate corrective response. Most mills will consider pumping back an effluent stream with a conductivity of over 5000 ?mhos to the black liquor system. This corresponds to a black liquor concentration of about 0.5%.  At some mills, the set point is as low as 2500 ?mhos [1]. Low discharge of colour or COD is an indication of good spill control. As a rough guide, a mill with O2 delignification that has below 40 kg colour/t or 30 kg COD/t in its discharges before treatment would have good spill control [1]. 

The best strategy is to prevent spills whenever possible. Appropriate instrumentation and knowledgeable mill staff are key factors. However, prevention is not always possible, and spill recovery sumps with automatic activation are required in critical areas [2]. A good spill control system provides operators with continuous, updated data on key parameters. The data must be understood by the operators, for them to be able to diagnose causes and take corrective action. Much of  the data required are the same as is needed to run an efficient operation; additional information, such as the levels of all major tanks, overflow alarms, and conductivity in floor drains, are also required. Continuous clean water discharges must be kept separate from the floor drains in areas covered by spill recovery. This is because the water dilutes the spills, and increases the black liquor evaporation load. 

1.       McCubbin, N., “Spill control: Assessing your situation”, Solutions!, 49-50, November, 2001.

2.     McCubbin, N., “Spill control, Part II: Reducing spills”, Solutions!, 36-37, December, 2001.